The specific subpopulations of neurons in the cat retina which utilize particular chemical neurotransmitters are being characterized with respect to their frequency of occurence, morphological appearance, and synaptic relationships. Light and electron microscopic autoradiography and immunocytochemical techniques are being used to identify subpopulations which utilize glycine, GABA, dopamine, and acetylcholine as transmitter substances. The identification and morphological characterization of these chemically defined subpopulations provides a basis for the development of a comprehensive wiring diagram of the organization of this mammalian retina.